Pete Tillman
Bluff with fault, Lospe fm(?) or older Quaternary gravels?
Bedrock under the gravels is the San Simeon ophiolite complex, a dike and sill complex which represents (I think) an old spreading center on the Jurassic(?) seafloor, as basaltic dikes were injected into the spreading "crack" to make new ocean floor. Sea-floor spreading was the key discovery in the then-new global tectonics which has evolved into the present-day understanding of plate tectonics, the details of which are way above my [retired] geologic pay-grade. Here, the diagnostic rock-type is microdiorite, which is simply finer-grained diorite: coarser than diabase, which is also reported to be present, but which I haven't yet seen. As you can see in the photo, the cobbles are sorted and crudely bedded, meaning they were deposited in fast-moving water. The yellow-brown color, from iron oxides, means deposition was subaerial = non-marine. I'm not sure what the story is for the larger, rounded "bowling-ball" cobbles below the bluff, except that they get rounded in the pounding surf, and are obviously strong! And not so easy to examine in person, for an old codger.
Structure: at the center of the photo, you will see a break (fault) with the right (south) side down. The unnamed fault displaced both bedrock and the older gravels. It was unmapped in the 1970s-era USGS mapping. It is likely a minor splay off one of the many faults which are more-or-less parallel to the coast, all likely related to the currently-active San Andreas fault system further east. According to my geologic guidebook, SLO county is moving North at around 1.5 inches a year. Which will bring us adjacent to downtown San Francisco in about 5 million years.... 🙀 Better shopping but worse traffic!
To the north is Point Sierra Nevada (rounded hill above it) and beyond that, the Santa Lucia mountains meeting the ocean about 15 miles north of here.
Bluff with fault, Lospe fm(?) or older Quaternary gravels?
Bedrock under the gravels is the San Simeon ophiolite complex, a dike and sill complex which represents (I think) an old spreading center on the Jurassic(?) seafloor, as basaltic dikes were injected into the spreading "crack" to make new ocean floor. Sea-floor spreading was the key discovery in the then-new global tectonics which has evolved into the present-day understanding of plate tectonics, the details of which are way above my [retired] geologic pay-grade. Here, the diagnostic rock-type is microdiorite, which is simply finer-grained diorite: coarser than diabase, which is also reported to be present, but which I haven't yet seen. As you can see in the photo, the cobbles are sorted and crudely bedded, meaning they were deposited in fast-moving water. The yellow-brown color, from iron oxides, means deposition was subaerial = non-marine. I'm not sure what the story is for the larger, rounded "bowling-ball" cobbles below the bluff, except that they get rounded in the pounding surf, and are obviously strong! And not so easy to examine in person, for an old codger.
Structure: at the center of the photo, you will see a break (fault) with the right (south) side down. The unnamed fault displaced both bedrock and the older gravels. It was unmapped in the 1970s-era USGS mapping. It is likely a minor splay off one of the many faults which are more-or-less parallel to the coast, all likely related to the currently-active San Andreas fault system further east. According to my geologic guidebook, SLO county is moving North at around 1.5 inches a year. Which will bring us adjacent to downtown San Francisco in about 5 million years.... 🙀 Better shopping but worse traffic!
To the north is Point Sierra Nevada (rounded hill above it) and beyond that, the Santa Lucia mountains meeting the ocean about 15 miles north of here.